On the verge of extinction an alien race that needs human blood to survive gives the world an ultimatum that could ultimately end humanity. A rebel group is determined to save civilisation and takes the attack to the aliens.

Phoebe and Paige search for Leo after Paige accidentally causes Piper to have amnesia, but the sisters soon learn he is being held on the mythical Island of Valhalla.

Carter pairs with Warrick in a race where more is at stake than a lucrative contract with a leading technology corporation.